1. Field of the Invention
The present invention relates generally to the art of measuring the flow rate of a fluid traversing a conduit. More particularly, the invention relates to techniques for ascertaining flow rate based on selected pressure differentials measured within the conduit.
2. Description of the Prior Art
Various laboratory and industrial situations often require accurate measurement of fluid flow. Such fluid flow is frequently determined based on a pressure differential measured in a conduit through which the fluid is passing. Many flowmeters utilizing pressure differential principles are energy conversion devices in which flow rate is determined based on a static-pressure drop across a restriction in the flow path. Examples of this type of flowmeter include the orifice plate, flow nozzle and venturi meter.
With each of these meters, the fluid will experience an increase in velocity as it passes through the restriction. Due to energy conversion, the increasing kinetic energy of the fluid concomitant with the increase in velocity will be offset by a reduction in potential energy. This reduction in potential energy will be evidenced by a drop in fluid static-pressure. Knowing the static-pressure drop, as well the density of the fluid and the flow area of the restriction in the flow path, a theoretical value of the flow rate can be determined. To compensate for viscous effects and other factors which tend to make actual flow rate other than ideal, the theoretical flow rate is typically multiplied by an empirical "discharge coefficient" to obtain the actual flow rate.
A pitot-static tube makes a measurement at a single point in the flow from which the local velocity can be calculated. This local velocity is determined using a pitot tube and a static-pressure tap on the surface of an outer tube enclosing the pitot tube. The pitot tube, which is an open-ended tube facing the flow, feeds a pressure measuring device but does not let fluid flow through itself. The pitot tube therefore presents a barrier to the oncoming flow at which the flow comes to rest. The area at the open end of the pitot tube is thus referred to as a "stagnation point." With careful design, the stagnation point gives total-pressure, which in the case of incompressible flow is generally the sum of static-pressure and dynamic-pressure (i.e., pressure due to kinetic energy of the flow). In the case of compressible flow, however, the equations are more complicated and total-pressure cannot be thought of simply as the sum of static and dynamic pressure.
Because a pitot-static tube determines local velocity at a single point in the flow, it is not usually thought of as a flowmeter. However, by making measurements at a grid of points, in a plane normal to the flow direction, the flow rate can be determined by integrating over the points in the grid. This is called a "survey." Then, by calibrating the result (i.e., the flow rate) versus the velocity measurement at a designated point (e.g., the flow centerline) for a range of flow rates, the flow rate in can be determined in the future by making a measurement at that point only. In this sense, a pitot-static tube can become a "flowmeter." Use of a pitot-static tube as a flowmeter, however, is often undesirable because flow distortions which can occasionally be introduced upstream of the measurement location may cause significant errors in the measured flow rate.